Metal substrates are usually made of a single metal, a single metallic alloy, a single metal matrix composite (MMC) or a combination of clad metallic film layers. Metal substrates made of metal matrix composites, in particular, offer broad design flexibility because an MMC is a composite that contains at least two metals, one of which is a substantially higher melting refractory metal or reinforcement compound. A substrate that is an MMC can be tailored to control certain physical characteristics, such as mechanical, thermal, electrical, and chemical properties by varying the relative proportions of the two or more metals to satisfy end use specification requirements.
Metal substrates have long been used in microelectronic packaging to house circuit chips, dice, electrical components, and the like. In such applications, one or more heat-generating components are mounted on a metal substrate that is conventionally termed a flange or a carrier substrate. Once these components are mounted, the carrier substrate is usually mounted on a printed circuit card or circuit board. Metallic carrier substrates are good electrical conductors and are especially suited to house components or chips that require electrical grounding. For example, an application in which chip grounding is necessary is laterally diffuse metal oxide semiconductors (LDMOS) which house field effect transistors (FET).
When a carrier substrate is used for microelectronic packaging, the thermal conductivity (TC) and the coefficient of thermal expansion (CTE) are among the most important properties considered in the design of carrier substrates. The carrier substrate is generally made of a material having high thermal conductivity so that there is high heat transfer away from the heat-generating component. Microelectronic packages must also be dimensionally stable to prevent warping, delamination between the chip and the carrier substrate, or even cracking of the chip during thermal cycling. Thus, the carrier substrate must be designed such that its thermal expansion approximately matches or is slightly higher than the expansion of the chip. Typically, the CTE of a chip is about 2.8 ppm/.degree. C. for a silicon chip and about 5.6 ppm/.degree. C. for a gallium arsenide chip, for example.
A common problem in the design of microelectronic packages, however, is that material candidates having high thermal conductivity also have a high CTE. For example, copper has a thermal conductivity of about 400 W/mK, however, the CTE of copper is about 17 ppm/.degree. C. Thus, a metal substrate containing a high concentration of copper yields unacceptable results in most microelectronics applications. Compared to metal substrates made of a single metal or metallic alloy, MMC carrier substrates can be better designed to match the thermal expansion characteristics of the chip or other heat-generating component attached to the carrier substrate while also providing improved heat transfer. For example, copper/tungsten and copper/molybdenum composites are commonly used in electronic packaging applications and have a thermal conductivity that ranges from about 130 W/mK to about 180 W/mK depending on the copper content of the composite. This is considerably less than the thermal conductivity of copper.
While MMCs offer broad design flexibility, certain end use performance characteristics of metal substrates are compromised due to the homogeneity of the metal substrates in the x-y plane. It is desirable to provide a metal substrate that has at least two discrete portions of material compositions in the x-y plane with each material composition having distinct material properties. It is desirable to provide a metal substrate that has improved thermal conductivity to dissipate heat generated by a die situated in a localized area as well as a surrounding area that constrains the expansion of such localized area. It is also desirable to produce a metal substrate that can be used in microelectronic packaging to improve heat dissipation of chips and heat-generating components while maintaining dimensional stability and minimal warpage during thermal cycling. It is further desirable to produce a functionally graded metal substrate that achieves these objectives at a lower cost.